One of the simplest ways to convert linear motion to rotary motion is by use of the scotch yoke mechanism shown in the schematic of FIG. 2. Piston "P", on moving linearly from its phantom line position to its solid line position, causes a force F1 to be transmitted to a connecting pin CP by piston rod R. Said pin slidably fits within conventional slots S in the arms of a yoke assembly. Thus, the application of force F1 results in reactive forces F2 and F3. These forces vary with the angle "A" of the yoke Y. Force F2 moves the yoke arms, thereby causing the rotary motion of the yoke. Force F3 is the vertical reaction to the vertical component of F2. In most scotch yoke mechanisms, this latter force F3 induces a bending moment in the piston rod R, causing the rod to bind and/or causing the piston to rub the cylinder wall. These result in any or all of excessive seal wear, cylinder wear, and premature failure of the rod bearing.
The most common way to deal with this lateral loading is to stabilize the piston rod end, opposite the piston, with an additional bearing, as shown by bearing B2 in FIG. 3. This is, however, a partial solution at best. The piston will still have a tendency to bend like a beam supported at both ends. Such bending still results in the rod binding in the bearings and/or the piston rubbing the cylinder wall. This invention addresses the problem of how to decouple the piston rod or rods from the lateral forces generated as a result of the yoke arm--yoke pin engagement.